(1)Field of the Invention
The present invention relates to an apparatus for measuring an amount of relative movement of a first member and a second member that are oppositely disposed so as to be relatively movable using an optical signal, an electric signal, or a magnetic signal.
(2)Prior Art
Conventionally, an optical encoder has a main scale on which a grating is formed. The grating is composed of light transmitting portions and light non-transmitting portions (or light reflecting portions and light non-reflecting portions) that are arranged at a predetermined pitch in such a manner that the ratio of the widths thereof is 1:1. Such a grating that each of the light transmitting portions or light reflecting portions has a rectangular pattern is, hereinafter, referred to a "rectangular grating". In a light transmission-type optical encoder, collimated light is radiated to such a main scale. The transmitted light of a rectangular pattern is emitted to an index scale having a rectangular grating which is composed of light transmitting portions and light non-transmitting portions that are arranged at a predetermined pitch. By detecting a bright/dark pattern of the transmitted light of the index scale that is modulated by the relative movement of the main scale and the index scale, a displacement signal that periodically varies can be obtained. By processing the displacement signal, the amount of displacement, namely the distance, can be measured.
The displacement signal obtained by the above-described optical encoder is theoretically a triangular wave that corresponds to the variation of the overlap of the scale gratings. However, in reality, due to an influence of diffraction of light or the like, a pseudo sine wave signal is obtained. The pseudo sine wave has too large harmonic distortions to treat as a sign wave. The harmonic distortions restrict the detection accuracy and resolution. In particular, when the grating pitch of the scales are as small as several ten .mu.m, It is known that the fluctuation of the gap between the main scale and the index scale becomes large and that even small gap fluctuation on the order of several ten am causes the distortion component to largely vary. Thus, it will be difficult to adjust the gap.
To solve such a problem, so far, several methods for reducing the harmonic distortions of the above-described optical encoder have been proposed. For example, (1) method using a sine wave grating of which each of the light transmitting portions of the index scale is formed in a sine wave shape (for example, U.S. Pat. No. 4,782,229), (2) method using a pair of rectangular gratings that have different phases for canceling odd order harmonic distortions (for example, Japanese Patent Laid Open Publication 3-48122), and (3) method setting the ratio of the widths of light transmitting portions and light non-transmitting portions of the index scale so as to cancel the harmonic distortions (for example, Japanese Patent Laid Open Publication 7-146160).
However, in the methods (1) to (3), only the index scale grating on the light receiving side is modified so as to reduce the harmonic distortions. Thus, the main scale is not modified. The transmitted light amount is restricted by the main scale. In particular, in the case that the method for detecting a bright/dark pattern of geometrical light is used, when the scale grating is finely formed, it is difficult to remove the influence of diffracted light and adjust the gap. Thus, a satisfactory S/N ratio cannot be obtained. In addition, in the method (1), it is difficult to finely form the scale gratings in comparison with the method using the rectangular grating.
This problem also applies to an electrostatic capacitance type encoder that detects the variation of a transferred pattern of an electric signal with electrostatic capacity coupling between electrodes of the scales and to a magnetic encoder that measures the displacement with magnetic coupling between the scales as well as the optical encoder. For example, in the electrostatic capacitance type encoder, due to an influence of an edge effect that cannot be represented by parallel plates, the wave distortion varies corresponding to the gap.